3-enoyltetronates and -tetramates as occurring in natural products, the 1′-methoxy isomers 8 and 9 are typically unreactive bisacyl ylides. This was rationalised by an X-ray structural analysis and abinitio calculations. However, the formal Wittig products of 8 and 9 are accessible in two steps via Wittig olefination of ylides 4/5 followed by 1′-O methylation of the resulting alkenes.